In use of microscopes, the eyes of the user are often subjected to light conditions which differ greatly. For example, different contrasting methods, such as bright-field, dark-field and phase-contrast, give different image brightnesses, and the brightness of the image also changes as a function of the current numerical aperture of the objective when switching lenses, e.g., when changing from one to another objective. These brightness changes occur suddenly, upon making the lens change. In addition, the eye can be exposed to other light flashes, as when indexing the lens turret or when switching incident-light reflectors, in that light can impinge on the barrels of the interchangeable optics and can thus be reflected directly into an eyepiece.
Such switching processes are dazzling to the observer. To avoid this problem, the observer ordinarily, before switching, turns down the potentiometer for brightness control of microscope illumination, and then carefully turns it up again. But this procedure is cumbersome and must be acquired as a habit, and it does not definitely prevent the observer from being dazzled by excessive brightness.
From European Patent No. EP-Al-0 124 241, a device is known for controlling brightness in a microscope. The known device employs a computer which calculates optimal image brightness from data (such as diaphragms and other factors) for the objective which is currently in operating position, and controls the source of light and additional filters accordingly. But this device does not afford any protection against light flashes which are attributable to switching processes.
In West German Patent No. 3,222,915, an automatic brightness control is described for the image of a mark which is reflected into the ray path of the microscope. This automatic system contains a sensor in the viewing-ray path and, by intensity control of an auxiliary source of light, adapts the brightness of the mark to the brightness of the image in the viewing-ray path. However, the microscope illumination itself is not affected by this automatic system; neither is there anti-dazzle protection for switching processes.